A key limiting factor in the widespread adoption of proton exchange membrane fuel cell (PEMFC) based power systems is hydrogen fuel storage. The development of a viable hydrogen storage solution will have a profound impact on how consumers will power portable devices, since batteries simply cannot match demands for runtime, energy density and reliability.
Because hydrogen has poor energy content per volume (0.01 kJ/L at STP and 8.4 MJ/L for liquid hydrogen vs. 32 MJ/L for petroleum), physical transport and storage as a gas or liquid is impractical. Additionally, the compression process to achieve the pressures necessary to reach a high density is energy-intensive and doesn't solve the hazard issue. Also, the densities of compressed H2 or liquefied H2 are still below those required to reach practical fuel storage goals.
Physical means to store hydrogen include sorbents such as carbon nanotubes and foams, zeolites, metal-organic frameworks; and intermetallics such as titanium-manganese alloy 5800, complex hydrides such as metal alanates, amides, and borohydrides, and chemical hydrides such as sodium borohydride/water and ammonia borane (AB). Despite intensive and elegant work on sorbents and complex hydrides, practical systems that can store and release ≧6 wt % hydrogen at moderate temperatures are still far from realization.
Alane is an attractive candidate for solid hydrogen storage and release because it has a density of 1.48 g/cm3 and releases up to 10 weight percent hydrogen and aluminum in a single step upon heating to ≧200° C. Alane's formula is sometimes represented with the formula (AlH3)n because it is a polymeric network solid. Alane is formed as numerous polymorphs: the alpha (α), alpha prime (α′), beta (β), delta (δ), epsilon (ε), zeta (ζ), or gamma (γ) polymorphs. Each of the polymorphs has different physical properties and varying stability. The most thermally stable polymorph is α-alane, featuring aluminum atoms surrounded by six hydrogen atoms that bridge to six other aluminum atoms. The Al—H distances are all equivalent and the Al—H—Al angle is approximately 141°. While α-alane's crystals have a cubic or hexagonal morphology, α′-alane forms needlelike crystals and γ-alane forms a bundle of fused needles. Typically, the lightweight, unstable γ-alane is produced first, converting under certain conditions to the more stable rhombohedral β-alane polymorph first, then to α-alane. When trace amounts of water are present during crystallization the δ-alane and ε-alane can be formed. The ζ-alane polymorph is prepared by crystallization from di-n-propyl ether. The α′, δ, ε, and ζ polymorphs do not convert to α-alane upon heating and are less thermally stable than α-alane.
Crystalline alane has many uses including: hydrogen storage, inorganic and organic synthesis, as an ingredient in propellants and pyrotechnics, as a polymerization catalyst, and as a precursor to aluminum films and coatings. Consequently there has been considerable research carried out on the preparation of alane, since the first report of its preparation in 1942 (Stecher and Wiberg, Ber. 1942, 75, 2003). Finholt, Bond, and Schlesinger reported an improved method of synthesis of alane-diethyl etherate in 1947 which has formed the foundation for most of the reported methods for the synthesis of non-solvated crystalline alane (J. Am. Chem. Soc., 1947, 69, 1199). The reaction is shown below, and the amount of ether complexed to the alane product depended on the length and temperature of the drying step of the reaction.3LiAlH4+AlCl3→4AlH3+3LiCl
Reports describing the preparation and stabilization of non-solvated crystalline alane began to appear in the patent literature in 1974 (Scruggs, U.S. Pat. No. 3,801,657, Roberts et al. U.S. Pat. No. 3,803,082, King, U.S. Pat. No. 3,810,974, Matzek et al. U.S. Pat. No. 3,819,819, Daniels et al. U.S. Pat. No. 3,819,335, Roberts, U.S. Pat. No. 3,821,044, Brower et al. U.S. Pat. No. 3,823,226, Schmidt et al. U.S. Pat. No. 3,840,654, and Self et al. U.S. Pat. No. 3,844,854). Removal of the residual diethyl ether (“desolvation”) was effected by using higher than stoichiometric ratios of complex aluminum hydride to aluminum chloride, as well as inclusion of lithium borohydride as a “seeding” or “crystallization” agent. Several patents describe the use of sodium aluminum hydride instead of lithium aluminum hydride (Ashby et al. U.S. Pat. No. 3,829,390, and Kraus et al. U.S. Pat. No. 3,857,930). As disclosed in these patents and Brower et al. (“Brower”), “Preparation and Properties of Aluminum Hydride,” J. Am. Chem. Soc., 1976, 98, 2450, alane is usually synthesized by reacting aluminum trichloride (AlCl3) and metal aluminum hydride (MAlH4) in diethyl ether or diethyl ether-hydrocarbon solvent mixtures. The aluminum trichloride was dissolved in diethyl ether at −10° C. A minimum of three mole equivalents of MAlH4 was added to the aluminum trichloride solution to produce a solvated alane-ether complex and a precipitate of metal chloride (MCl, e.g. LiCl or NaCl). In order to desolvate the alane-ether complex, 0.5 to 4.0 mole equivalents of a borohydride salt, such as lithium borohydride or sodium borohydride, was mixed with the solution including the alane-ether complex. The mixture was filtered and the filtrate was diluted with toluene or benzene to provide an ether to toluene or benzene ratio of 15:85. The mixture was heated to 85° C. to 95° C. to desolvate the alane-ether complex and the diethyl ether was subsequently removed by distillation. The precipitated alane was recovered by aqueous acid quenching, filtration, and washing. Brower also discloses that the reaction is conducted in the absence of water, oxygen, and other reactive species because if water is present, the δ and ε polymorphs are undesirably formed.
The methods reported for stabilization of the reactive alane product during this time included in situ or subsequent treatment of alane with an alkyl or aryl silicol, coating the alane surface with an organic compound containing at least one phenyl group or a condensed ring structure, and washing the alane product (often with some amount of magnesium included in the preparation step) with an aqueous solution buffered at from about pH 6 to 8.
However, the large volumes of solvent required as well as the excess aluminohydride and borohydride salts used to desolvate the alane-ether complex make these syntheses of α-alane expensive. The borohydride salts also generate byproducts that require disposal. Furthermore, the alane produced by the method of Brower is typically contaminated with undesirable polymorphs and is prone to decomposition during desolvation.
Current methods for the preparation of alane are expensive because of, among other things, the high cost of the large amounts of solvent needed to prepare the stable α-alane crystalline phase. It would be desirable to reproducibly produce a high yield of α-alane using a low-cost method.
An object of the present invention is to provide an improved low-cost method for the preparation of α-alane suitable for use as a solid hydrogen storage and release material. In addition, alane-etherate can be used as a reducing agent in organic synthesis and as an initiator in polymerization reactions.